How Vaccines Work

Vaccines are usually given via a hypodermic injection, but some are given through the mouth or nose. There are two main groups of vaccines: live-attenuated vaccines and inactivated vaccines.

Live-attenuated vaccines: Live-attenuated basically means alive, but very weak. These vaccines are made when the virus is weakened to such a level that they reproduce only about 20 times in the body. By comparison, natural viruses reproduce thousands of times. When the vaccine is made, the virus or bacteria is weakened in a laboratory to the point where it's still alive and able to reproduce, but can't cause serious illness. Its presence is enough to cause the immune system to produce antibodies to fight off the particular disease in the future.

"Live-attenuated vaccines can cause very mild illness in a small proportion of people," says John Bradley, M.D., member of the committee on infectious diseases of the American Academy of Pediatrics (AAP). "However, these side effects are usually very mild and limited to a low-grade fever or runny nose." Dr. Bradley also notes that about 5 to 10 percent of children who receive the varicella (chickenpox) vaccine develop a few pox spots, but it's nothing compared to the full-blown illness.

To weaken the virus, scientists must isolate it through a specimen from an infected person. They then grow the virus in a test tube. They "pass" the virus into a second test tube, then a third, a fourth and so on. Scientists perform this "passage" many times -- the measles virus was passed 77 times! The virus is periodically taken out of the test tube to see if it has mutated. Eventually, the virus gets so used to living in the comfortable test-tube environment that it loses its capacity to produce illness in humans. These passages are performed in a very controlled environment in exactly the same way each time. This discovery was considered the "hallelujah" of vaccine development, according to William Schaffner, M.D., professor and chair of the Department of Preventive Medicine at Vanderbilt University School of Medicine.

Examples of live-attenuated vaccines are MMR (measles, mumps and rubella combination vaccine), varicella and the intranasal form of influenza.

Vaccine Side Effects

As with any drug, there's always the potential for side effects. Vaccine side effects are generally pretty mild and limited to soreness at the injection site, headache or a low-grade fever. Serious reactions aren't completely impossible, though they are rare. If you suspect a severe reaction, call the doctor immediately to have it evaluated. Information about possible reactions to specific vaccines is available here.

Inactivated vaccines: When inactivated vaccines are made, the bacteria is completely killed using a chemical, usually formaldehyde. Dead pieces of disease-causing microorganisms (usually bacteria) are put into the vaccine. Because the antigens are dead, the strength of these vaccines tend to wear off over time, resulting in less long-lasting immunity. So, multiple doses of inactivated vaccines are usually necessary to provide the best protection. The benefit of inactivated vaccines is that there is zero chance of developing any disease-related symptoms -- allergic reactions are possible but extremely rare.

Why are some vaccines live and some dead?"The bottom line is that the decision is entirely driven by the science," says Dr. Schaffner. "If scientists can make a killed vaccine that is effective, that is what they will do. It's all about trial and error." Most viral diseases, he says, require live-attenuated vaccines, but the vast majority of bacterial illnesses are prevented with inactivated vaccines. There are some exceptions to this rule, though. For example:

Some travelers to less-developed countries get the vaccine to prevent typhoid fever. There are live and killed forms of this vaccine.

Rabies is a viral infection that is 100 percent fatal once it has progressed. The disease is simply too dangerous to give, even in a weakened state. Fortunately, science allowed the development of an inactivated rabies vaccine.

So what, exactly, are the ingredients of a vaccine? Read on to find out.